1. Field of the Invention
The present invention relates to a thermoelectric conversion element, a thermoelectric conversion module, and a method for producing a thermoelectric conversion element.
2. Description of the Related Art
To prevent global warming, the reduction of carbon dioxide is a critical issue. Thermoelectric conversion elements capable of directly converting heat into electricity have recently been receiving attention as one effective technique of utilizing waste heat.
For example, as shown in FIG. 6, a thermoelectric conversion element 50 including a p-type thermoelectric conversion material 51, an n-type thermoelectric conversion material 52, lower-temperature-side electrodes 56, and a higher-temperature-side electrode 58 is known as a conventional thermoelectric conversion element.
In the thermoelectric conversion element 50, two types of thermoelectric conversion materials 51 and 52 are provided for energy conversion between heat and electricity and are connected to the lower-temperature-side electrodes 56 at lower-temperature-side junctions 53b, which are defined by end surfaces on a lower-temperature side. Furthermore, the thermoelectric conversion materials 51 and 52 are connected to each other at higher-temperature-side junctions 53a, which are end surfaces on a higher-temperature side, with the higher-temperature-side electrode 58. In the thermoelectric conversion element 50, the application of a temperature difference between the higher-temperature-side junctions 53a and the lower-temperature-side junctions 53b generates an electromotive force caused by the Seebeck effect, thereby providing electricity.
However, in the thermoelectric conversion element 50, the electrodes 56 and 58 are used to connect the two thermoelectric conversion materials 51 and 52, thereby disadvantageously producing contact resistance between the electrodes and the thermoelectric conversion materials.
The electric-generating capacity of a thermoelectric conversion element is determined by thermoelectric conversion characteristics of a material and a temperature difference applied to the element, and is also significantly affected by the occupancy of the thermoelectric conversion materials, that is, by the proportion of the area of the thermoelectric conversion materials in a plane perpendicular to the direction of the temperature difference applied to the thermoelectric conversion element. A higher occupancy of the thermoelectric conversion materials leads to an increase in the electric-generating capacity of the thermoelectric conversion element per unit area.
However, in an exemplary conventional structure, such as the thermoelectric conversion element 50, a gap insulation layer is provided between the two thermoelectric conversion materials 51 and 52. Thus, the extent to which the occupancy of the thermoelectric conversion materials can be increased is limited.
Furthermore, since the insulation gap is provided between the two thermoelectric conversion materials 51 and 52, the element is susceptible to damage due to impact by, for example, dropping. Thus, the element disadvantageously has low reliability.
As described above, a higher occupancy of the thermoelectric conversion materials is desirable to increase the electric-generating capacity of the thermoelectric conversion element. As a method for overcoming the foregoing problems, a thermoelectric conversion element including p-type and n-type thermoelectric conversion materials that are directly bonded has been disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 8-32128 and Japanese Unexamined Patent Application Publication No. 2002-118300).
In the direct-junction thermoelectric conversion elements disclosed in Japanese Unexamined Patent Application Publication No. 8-32128 and Japanese Unexamined Patent Application Publication No. 2002-118300, the p-type and n-type thermoelectric conversion materials are directly bonded, and there is no need to provide a gap therebetween. Thus, the occupancy of the thermoelectric conversion materials is increased.
That is, Japanese Unexamined Patent Application Publication No. 8-32128 discloses a thermoelectric conversion element in which p-type and n-type thermoelectric conversion materials are alternately stacked, the p-type and n-type thermoelectric conversion materials are electrically connected, and an insulating layer is arranged in a region other than a junction region of a stack interface. The insulating layer is formed by firing a mixed material including at least one insulating ceramic materials selected from the group consisting of ZrO2, Al2O3, MgO, TiO2, and Y2O3 and a glass containing SiO2, B2O3, Al2O3, and an alkaline-earth metal oxide, the glass content of the mixed material being in the range of 10% to 50% by weight. The p-type and n-type thermoelectric conversion materials are obtained by firing a modified iron silicide (FeSi2).
However, in the thermoelectric conversion element disclosed in Japanese Unexamined Patent Application Publication No. 8-32128, an iron silicide (FeSi2)-based material is used as the thermoelectric conversion material. Thus, a special firing method in which firing is performed in a vacuum is required, which disadvantageously increases the cost and complexity of the production process. Furthermore, the iron silicide-based material has a high thermal conductivity, which disadvantageously causes difficulty in applying a temperature difference to the thermoelectric conversion element. Moreover, the iron silicide-based material may be deteriorated by oxidation occurring at a high temperature.
Another thermoelectric conversion element is disclosed in which p-type and n-type semiconductor materials are bonded by charging at least two different oxide semiconductor powders into a die so as to form at least two layers and performing spark plasma sintering under pressure (see, for example, Japanese Unexamined Patent Application Publication No. 2002-118300).
In the thermoelectric conversion element disclosed in Japanese Unexamined Patent Application Publication No. 2002-118300, although oxide thermoelectric conversion materials are used as the thermoelectric conversion materials, for example, a modified material of ZnO having a wurtzite structure is used as the n-type thermoelectric conversion material, and NiO having a tetragonal structure is used as the p-type thermoelectric conversion material. They have different sintering temperatures. Thus, it is necessary to perform sintering under pressure. Similar to Japanese Unexamined Patent Application Publication No. 8-32128, there are problems of increased cost and complexity of the production process.
In addition, a thermoelectric material having a composition of the chemical formula (La1-xBax)2CuO4, (La1-xSrx)2CuO4, or (Y1-xBax)2CuO4 is reported (Japanese Examined Patent Application Publication No. 6-17225) as a thermoelectric conversion material defining a thermoelectric conversion element, wherein x is in the range of 0<x<1. Japanese Examined Patent Application Publication No. 6-17225 also discloses that the firing of the thermoelectric material is performed at 1100° C. for 5 hours.
Furthermore, a thermoelectric conversion material formed by doping a complex oxide of the chemical formula Nd2CuO4 with Zr or Pr is disclosed as a thermoelectric conversion material defining a thermoelectric conversion element (see, for example, Japanese Unexamined Patent Application Publication No. 2000-12914). Japanese Unexamined Patent Application Publication No. 2000-12914 also discloses that the firing of the thermoelectric material is performed at 1100° C. for 10 hours.
In the conventional techniques disclosed in Japanese Examined Patent Application Publication No. 6-17225 and Japanese Unexamined Patent Application Publication No. 2000-12914, the compositions and the firing conditions of the oxides defining the thermoelectric conversion materials are described. However, with respect to a combination of the n-type and p-type thermoelectric conversion materials and a method for directly bonding the n-type and p-type thermoelectric conversion materials, which are required to produce a small, high-performance thermoelectric conversion element, no specific description is provided.